1. Field of the Invention
The present invention relates generally to medical catheter devices, and more particularly to tissue ablation catheters having expandable distal end assemblies. The predominant current usage of the inventive ablation catheter with collapsible electrode is in the field of medical catheter surgery in applications wherein it is desirable to produce a lesion of substantial size.
2. Description of the Prior Art
It is known in the medical field that an important method for correcting cardiac fibrillation is to interrupt unwanted potential electrically conductive pathways or "reentry circuits" whereby electrical impulses are misdirected in the heart. This has been done surgically using open heart surgery methods. However such methods are expensive and technically difficult and are, therefore, only rarely attempted. An alternative, and generally superior method has been to accomplish the same result using electrical ablation with the electrodes being introduced into the heart by means of a catheter. A method and means for accomplishing this which is particularly adapted for treating the atria in order to correct atrial fibrillation is taught in U.S. Pat. No. 5,549,661 issued to Kordis et al.
It has been found, however, that related treatment of the ventricles, and of certain other areas of the body as will be discussed in more detail hereinafter, requires a substantially larger area of ablation than is practical using the methods and means described in the Kordis et al. patent referenced above. The largest sized catheter tip to be reliably used intravascularly is referred to as size 8 French. It is known that an 8 French ablation tip generally does not create large enough lesions to ablate VT substrates, especially those positioned intramural or on the epicardial surface. Other technologies including microwave, laser, ultrasound, and chemical ablation are unproven and will require an extensive development process. Fluid cooling technology using RF power and currently available tip sizes increases lesion size by forcing the maximal temperature deeper into the tissue, but this may be accompanied by complications due to "popping" and the resulting thrombi.
Current RF ablation technology has constraints to its lesion creating capabilities. Lesion size is limited by the surface area of the electrode, with theoretical improvements made through fluid cooling the electrode and applying more power through the cooled electrode to force the region of hottest tissue temperature deeper into the tissue. Since cardiac catheter ablation depends on introducing the catheter through preferably an 8F introducer and advancing the ablation catheter tip through the vasculature into the desired heart chamber, size is an important consideration when designing an RF ablation catheter.
It would be desirable to have an ablation catheter which could reliably and safely produce lesions of a size more desirable for the treatment of ventricular conditions. However, to the inventors' knowledge, no such catheter has existed in the prior art. All prior art ablation devices capable of producing the desired lesions have either been too large to introduce through a catheter, or else have required manipulation by the operating physician across the area to be ablated--this last being both difficult and otherwise generally undesirable.
Expandable catheter end portions are known in the art. An example of such is taught in U.S. Pat. No. 5,293,869 issued to Edwards et al., wherein an expandable cardiac probe is used for the purpose of maintaining constant contact with the endocardial surface during expansion and contraction thereof. Balloon catheters are also known in the art, these being used as a general means for enlarging the distal end of a catheter as required. However, to the inventors' knowledge, no catheter having an expandable electrode portion suitable for producing enlarged lesions has been known in the prior art.